Combustion misfires lead to an increase of toxic substances emitted during operation of the engine and can, in addition, lead to damage of the catalytic converter in the exhaust-gas system of the engine. A detection of combustion misfires in the entire rpm and load ranges is necessary to satisfy statutory requirements as to onboard monitoring of exhaust-gas relevant functions. In this context, it is known that, during operation with combustion misfires, characteristic changes occur in the rpm curve of the engine compared to normal operation without misfires. Normal operation without misfires and operation with misfires can be distinguished from a comparison of these rpm curves.
A method operating on this basis is already known and disclosed in German patent publication 4,138,765 which corresponds to U. S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned.
In this known method, a crankshaft angular region which is characterized as a segment is assigned to a specific region of the piston movement of each cylinder. The segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time in which the crankshaft passes through this angular region is dependent, inter alia, upon the energy converted in the combustion stroke. Misfires lead to an increase of the segment times detected in synchronism with the ignition. According to the known method, a criterion for the rough running of the engine is computed from the differences of the segment times. In addition, slow dynamic operations such as the increase of the engine rpm for a vehicle acceleration are mathematically compensated. A rough-running value which is computed in this way for each ignition, is likewise compared ignition-synchronously to a predetermined threshold value. Exceeding this threshold value is evaluated as a misfire. The threshold value is dependent, as may be required, from operating parameters such as load and engine speed (rpm).
The reliability of the method is dependent decisively upon the precision with which the rpm of the crankshaft can be determined from the segment times. The segment time determination is dependent upon the accuracy with which the markings can be produced on the transducer wheel during manufacture. These mechanical inaccuracies can be mathematically eliminated. For this purpose, it is known from U.S. Pat. No. 5,428,991 to form, for example, three segment times per crankshaft revolution during overrun operation. One of the three segment times is viewed as a reference segment. The deviations of the segment times of the two remaining segments to the segment time of the reference segment are determined. From the deviations, corrective values are so formed that the segment times are the same with respect to each other. These segment times are determined in overrun operation and are logically coupled to the corrective values.
Segment times can be determined in normal operation outside of overrun operation and can be logically coupled to the corrective values. Deviations of these segment times are therefore independent of manufacturing accuracies of the transducer wheel and indicate other causes.
When misfires are recognized from the detected rpm trace, then additional influences on the rpm are to be considered which are not caused by misfires. An example of such influences to be considered are torsion vibrations which are superposed on the rotational movement of the crankshaft. These occur primarily at high rpm during fired operation and lead to a systematic increase or decrease of the segment times of individual cylinders so that the misfire detection is made more difficult. For this reason, and also for differences between individual engines (caused by wear or manufacturing inaccuracies), a base noise in the form of a spread of the segment times remains which cannot be attributed to misfires. Actual misfires are that much more difficult to distinguish from this base noise the fewer individual misfires act upon the rpm of the crankshaft. The reliability of the misfire detection therefore drops with an increase in the numbers of the cylinders of the engine and with increasing rpm as well as with decreasing load.